Using a scanning tunneling microscope, the differential conductance (dI/dV) versus the applied bias voltage (V) on Au(111) and n- and p-type Si(001) surfaces was measured under a constant tunneling current. STM tips were treated by a thermal-field treatment before measurements to improve the reproducibility of dI/dV-V curves. For both directions of electron tunneling from tip to sample and vice versa, oscillation phenomena in the dI/dV-V curves were found, owing to excitation of electron standing waves in a vacuum gap between a tip and a sample surface. For electron tunneling from sample to tip, oscillation peaks for n-type Si(001) were weakened near applied voltages corresponding to an averaged work function of sample and tip possibly due to electron emission from surface states. The features in the oscillation phenomena of the dI/dV-V curves are discussed on the basis of the energy band diagram.